1. Field of the Invention
This invention relates to the field of hydraulic systems. More particularly, it pertains to the source of supply of hydraulic fluid to the inlet of a hydraulic pump. The system is especially adapted for use in an automatic transmission for automotive vehicles having a low volume sump and a higher volume reservoir.
2. Description of the Prior Art
Automatic transmissions, particularly those used in front wheel drive vehicles, are susceptible to oil pump cavitation. This condition is caused when transmission fluid in the sump that collects hydraulic fluid from the control circuit of the transmission is thrown by centrifugal force away from the pump inlet in the sump when the car turns at high speed.
Various attempts have been made to overcome this difficulty. The scavenging system described in U.S. Pat. No. 4,762,201 uses a substantially dry sump from which hydraulic fluid is taken to fill a reservoir by means of a vacuum. Hydraulic fluid required by the transmission is directly pumped from the reservoir. The system provides no means to switch the pump inlet connection between the reservoir fluid and sump fluid in response to various operating conditions of the system.
U.S. Pat. No. 4,168,638 describes a fluid pressure supply system for a hydraulic control system of an automatic transmission. This system uses engine intake manifold as a vacuum source to supply hydraulic fluid to a reservoir from which fluid is supplied to produce gear shifting. The system includes a high pressure cutoff valve and an additional cutoff valve between the vacuum source and the supply source, which valves respond to engine operating conditions and movement of the gearshift mechanism of the transmission.
U.S. Pat. Nos. 4,599,979; 4,697,553; and 4,750,456 describe lubrication systems that use intake manifold vacuum to draw lubrication fluid from a reservoir or sump to lubricate an internal combustion engine. U.S. Pat. No. 4,681,189 describes a dry sump lubrication system for an internal combustion engine. That system is particularly directed to separating air from the pump oil in a dry sump system.